The present invention relates to lubricating oil compositions, such as automatic transmission fluids (ATF), containing a dual additive combination of two components to impart low temperature, low viscosity, and shear stability to the formulations containing the same.
Automatic transmission fluids typically are formulated to exhibit the proper viscosity at both high and low temperatures. Thus, at normal operating temperatures (85.degree. to 120.degree. C.), automatic transmission fluids must have a sufficiently high viscosity to prevent excessive leakage in the control and hydraulic systems. The fluid is subjected to shearing action by the transmission and must be shear stable in order to retain the advantages of this viscosity at elevated temperatures. On the other hand, automatic transmission fluids must also exhibit proper fluidity at low temperatures in order to insure proper functioning of pumps, prevent "burning" of clutch plates, and to allow free flow of fluid at temperatures of -30.degree. C. and lower. Thus, low viscosity favors high efficiency in converters, and operation at low temperatures will be sluggish or fail when viscosity is excessively high.
The property of a fluid to resist changes in viscosity due to changes in temperature can be expressed as the "viscosity index" (VI); an emperical unitless number. The higher the viscosity index of an oil, the less its viscosity changes with changes in temperature. The demands placed on automatic transmission fluids make an oil of high viscosity index highly desirable.
Accordingly, additives have been developed to increase the viscosity index of the lubricant and thus extend the versatility and lubricating qualities of the lubricant at both high and low service temperatures. However, not only are the viscosity index properties especially important to automatic transmission fluids, but so also are the thickening effects of viscosity index improving agents upon the lubricant base.
More specifically, one of the stringent requirements for automatic transmission fluids is the relatively narrow limit of viscosity values. For example, a DEXRON.RTM. II specification for automatic transmission fluids has strict requirements in respect to low temperature viscosity characteristics and places a maximum viscosity limit of about 50,000 centipoises (cps) at -40.degree. C. Such low temperature viscosity requirements must be achieved while simultaneously meeting high temperature viscosity requirements. Accordingly, many viscosity index improving agents conventionally added to automatic transmission fluids are associated with both low and high temperature effects. The low temperature viscosity effect is believed to result from interaction of the viscosity index improver with wax typically present in lubricants. Such interaction causes an initial decrease in low temperature viscosity at low level amounts of the viscosity index improver while simultaneously thickening the lubricant at more elevated temperatures. However, because there are limited amounts of wax present in conventional lubricants, increasing the amount of viscosity index improver beyond certain concentrations eventually exceeds the useful wax interactive amount, and the low temperature viscosity actually begins to increase. The result is that limits are placed on the amount of viscosity index improver concentration which can be added to achieve both low and high temperature viscosity benefits. Other materials are available which affect primarily only the low temperature viscosity properties such as pour point depressants. Pour point depressants also interact with wax to cause an initial decrease in low temperature viscosity. However, the effectiveness of such materials is also limited by the amount of wax present in the fluid for the same reasons discussed above in connection with the high and low temperature viscosity improving agents.
Accordingly, the search has continued for alternate ways to achieve low temperature viscosity requirements thereby permitting more flexibility in achieving both low and high temperature viscosity properties. The present invention is a result of this search.
U.S. Pat. No. 3,702,300 discloses multifunctional additives for lubricant compositions which are carboxy containing interpolymers in which many of the carboxy groups are esterified and the remaining carboxy groups are neutralized by reaction with polyamino compounds. Suitable carboxy containing polymers disclosed include interpolymers of maleic anhydride and styrene (Col. 4, Lines 32 et seq.). The additives are employed to impart anti-sludge properties and desirable viscosity properties to lubricants, particularly automatic transmission fluids (Col. 10 Lines 1 et seq.). Thus, this patent discloses a conventional viscosity index improver but does not disclose its combination with the materials of component B as described herein.
U.S. Pat. No. 4,240,916 is directed to oil soluble copolymers of 1-olefins and maleic anhydride and esters thereof and their use as pour point depressants in lubricating oils. However, styrene is not disclosed as a suitable 1-olefin. Similar copolymers are disclosed in U.S. Pat. No. 4,151,069 as dewaxing aids for lubricating oils.
U.S. Pat. No. 4,229,311 discloses a method for making polymer in oil solutions useful for improving the viscosity-temperature relationship and low temperature properties of a lubricating oil. In accordance with said method, a methacrylic acid ester of a C.sub.8 to C.sub.18 alcohol is polymerized onto a C.sub.2 to C.sub.4 olefin polymer in an oil solution of the polyolefin and ester monomers. Additional polyolefin is added to the solution and a nitrogen containing heterocyclic compound is graft copolymerized onto the resulting mixed backbone methacrylate and olefin copolymer. Alternatively, the methacrylate monomer can be graft co-polymerized onto the olefin polymer to form a backbone polymer onto which the heterocyclic compound is graft copolymerized.
A number of patents disclose additive combinations for middle distillate fuels which are distinct from lubricating oils but are included herein for background purposes.
Thus U.S. Pat. No. 3,762,888 discloses the combination of a pour point depressant for fuel oils in combination with auxiliary flow improving compounds. The pour point depressant can be an ethylene polymer, a hydrogenated olefin polymer, a C.sub.10 to C.sub.18 olefin polymer, a hydrogenated ethylene polymer, or an ethylene copolymer wherein the comonomer can be a vinyl ester such as vinyl acetate, an ethylenically unsaturated ester such as methylacrylate or methylmethacrylate, fumarates and maleates. The auxiliary flow improving compound contains at least one straight chain polymethylene segment containing a bulky substituent which may be polar or nonpolar. Representative auxiliary flow improvers disclosed include sorbitan monostearate, sorbitan tristearate, polyoxyethylene (8) stearate polyoxyethylene (20) sorbitan tristearate and the like.
U.S. Pat. No. 3,961,916 discloses a dual functioning flow improving composition, for middle distillate fuel oils, containing a wax growth arrestor and a nucleating agent. The wax growth arrestor can be an ethylene vinyl acetate copolymer and the nucleator can be a different ethylene vinyl acetate copolymer, the nucleator differing from the growth arrestor by the proportion of the vinyl acetate present therein and/or the molecular weight of the polymer. The combination imparts improvements in the filterability of middle distillate fuels.
U.S. Pat. No. 4,375,973 discloses a three component additive combination for improving cold flow properties cf middle distillate fuels. These three components are: (a) a distillate flow improver which is an ethylene containing polymer, typically a copolymer of ethylene with unsaturated esters, e.g , vinyl acetate; (b) a hydrocarbon polymer of C.sub.2 to C.sub.30 olefin of number average molecular weight of 10.sup.3 to 10.sup.6 or a derivatized version thereof, for example, copolymers of ethylene and propylene, or polyisobutylene, which are used as lubricating oil viscosity index improvers; and (c) a polar oil soluble compound which includes amides, salts, carboxylates, sulfonates, sulfates, phosphates, phenates and borates, having hydrocarbon solubilizing groups, for example, salts and amides of polycarboxylic acid such as phthalic anhydride reacted with hydrogenated secondary tallow amine.
U.S. Pat. No. 4,464,182 is directed to a narrow boiling distillate fuel oil containing certain polyalkylene esters, ethers, ester/ ether mixtures thereof. Such additives have at least two to C.sub.30 linear saturated alkyl groups and a polyoxyalkylene glycol derived moiety of a molecular weight of 100 to 5,000 present in the structure. The alkyl group of the polyoxyalkylene glycol contains from 1 to 4 carbons. In broad boiling distillate fuels the ester, ether, or ester/ether additives are typically employed in conjunction with other flow improver additives. Suitable additional flow improvers as disclosed include ethylene copolymers wherein the comonomer is an unsaturated mono- or di- ester such as vinyl acetate, methyl methacrylate and various fumurates.
British Patent Specification No. 711,364 is directed to a diesel fuel containing a small amount of polyethylene ether, or epoxy, derivative of a carboxylic acid ester of a polyhydric alcohol.
None of the above discussed patents disclose the particular additive combination as a low temperature viscosity modifier for lubricating oil compositions.